Chapter 2 Main Types of Organometallic Derivatives

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1 hapter 2 ain Types of rganometallic Derivatives Abstract This chapter describes the various functional derivatives that form the backbone of transition metal chemistry. In each case, the coordination modes of the involved ligand are presented, then the main synthetic routes, the reactivity, and the most useful analytical techniques are described. For metal hydrides, the more specific points concern the η 2-2 complexes and the influence of spectator ligands on the acidity basicity of hydrides in solution. For metal carbonyls, the high lability of the structures is stressed with its consequences for their analysis by I or 13 N. For metal alkyls or aryls, the various decomposition pathways are discussed with a special emphasis on the β- elimination of importance for polymerization catalysis. The section is completed by a presentation of the uses of the zirconium carbon bond in organic synthesis. The section on metal carbenes starts by a thorough discussion of the factors favoring the singlet or triplet ground states in free carbenes. Their complexation by transition metals leads to electrophilic (Fischer) or nucleophilic (Schrock) complexes with very different reactivities. Their role in the metathesis of alkenes is highlighted. A similar presentation of metal carbynes and their role in the metathesis of alkynes completes this section. The final section describes some specific π-complexes, η 4 -diene-irontricarbonyls, ferrocene and η 6 -arene-chromium-tricarbonyls which are widely used in organic synthesis. Keywords etal hydrides etal carbonyls etal alkyls etal carbenes π-complexes 2.1 etal ydrides For a long time, the existence of stable metal hydrides was controversial because the bond is highly reactive and was difficult to detect before the emergence of proton N spectroscopy. The main coordination modes of hydrogen are indicated below. F. athey, Transition etal rganometallic hemistry, SpringerBriefs in olecular Science, DI: / _2, The Author(s)

2 28 2 ain Types of rganometallic Derivatives Terminal (η 1 ) L n Bridging (µ 2) L n L n Bridging (µ 3 ) Ln Ln The bridging hydrides are considered as protonated bonds (μ 2 ) or protonated clusters (μ 3 ). The bond is always bent: angle between ca. 80 and 120. These bridging species are a little bit delicate to handle for the electron counts. The best way is to count the number of electrons of the L n units, count the bond if existing, then add the electrons of and other bridging ligands. Ln p() 2 o e P e o() 2 p o 6e e 2 P 3e 2e p 5e 1e ere, each metal unit has 15e, the o o bond counts for 1, the two bridging ligands share their 4e between the two molybdenum atoms. The other important case concerns the η 2-2 complexes discovered by Kubas in 1984 [1]. The bonding can be decomposed into a donation from the σ bond to the d z 2 empty orbital of the metal and a back donation from the d xz orbital of the metal to the σ* antibonding orbital of. In order to avoid the cleavage of, this backbonding must be suppressed or minimized. This can be achieved when using metal centers with low d electron counts and electron-accepting ancillary ligands like, etc. The existence of the bond in these complexes is established by I spectroscopy (the stretching frequency of η 2 - corresponds to a band around 2700 cm 1 ), by proton N (replacing by D and measuring the D coupling), and by neutron diffraction. Donation 2 to Back-donation to 2 The synthesis of hydrides is classical in most cases: oxidative addition of 2, reduction of l by LiAl 4 or other source of, protonation, hydrogenolysis of bonds, etc. A good example is the synthesis of the Schwartz reagent which is useful for the hydrozirconation of alkenes: LiAl 4 2 l 2 p 2 Zrl 2 p 2 Zr()l + p 2 Zr 2 p 2 Zr()l

3 2.1 etal ydrides 29 Apart from these classical routes, it is also possible to get hydrides using the so-called β- elimination. It operates with metal alkyls and metal alkoxides when β- is available, and also with metal formates and metal hydroxycarbonyls. 2 = 2 + -L n L n = + -L n L n This route from alkoxides works only with nonoxophilic metals, mainly from the platinum group (u, h, Pd, Ir, Pt), for example: K 2 Irl 6 Et + P 3 ( 3 P) 3 Ir Et l l -e ( 3 P) 3 Ir As already mentioned, the best method for detecting the bond is proton N spectroscopy. The hydride resonance appears between 0 and 50 ppm (e 4 Si), in a range which is empty for organic groups. The only exception is for d 0 and d 10 metal hydrides which resonate at low fields (δ positive). The bond is relatively strong; the bond dissociation energy (BDE) varies between 37 and 65 kcal mol 1. Generally, these hydrides are poorly soluble in water and not compatible with this solvent. Nevertheless, [o() 4 ] is an acid as strong as sulfuric acid whereas [e( 5 5 ) 2 ] is a base comparable to ammonia. It must be noted, however, that [o() 4 ] is a genuine hydride in the gas phase ( 0.75 e on ). ore general and precise data can be measured in acetonitrile as the solvent: Some pka in 3 N: [r() 3 p] 13.3 [o() 3 p] 13.9 [W() 3 p] 16.1 As can be seen, the hydride character increases with the atomic weight of the metal. [o() 4 ] 8.4 [o() 3 (P 3 )] 15.4 The influence of the other ligands on the metal is enormous. is more basic when L is a better donor. Some of the most useful reactions of metal hydrides are given hereafter: + l 4 l + l () + 2 = = 2 The third reaction is the reverse of the β- elimination. etal formyls [ ], which are the primary products of the insertion of into the bonds, are unstable in most cases but are important catalytic intermediates. l l

4 30 2 ain Types of rganometallic Derivatives 2.2 etal arbonyls etal carbonyls have been known since the end of the nineteenth century and are widely used in the chemical industry. A listing of the main stable metal carbonyls is given here. ost of them obey the 18e rule. A notable exception is vanadium hexacarbonyl which is a reactive blue radical which cannot dimerize for steric reasons. Several heavy clusters of osmium are also known [2]. [Ti() 6 ] 2 [V() 6 ] [r() 6 ] [n 2 () 10 ] [Fe() 5 ] [o 2 () 8 ] [Ni() 4 ] [V() 6 ] [Fe 2 () 9 ] [o 4 () 12 ] [Fe 3 () 12 ] [o 6 () 16 ] [Zr() 6 ] 2 [Nb() 6 ] [o() 6 ] [Tc 2 () 10 ] [u() 5 ] [h 2 () 8 ] [Pd() 4 ] 2+ [Tc 3 () 12 ] [u 2 () 9 ] [h 4 () 12 ] [u 3 () 12 ] [h 6 () 16 ] [f() 6 ] 2 [Ta() 6 ] [W() 6 ] [e 2 () 10 ] [s() 5 ] [Ir 2 () 8 ] [Pt() 4 ] 2+ [s 2 () 9 ] [Ir 4 () 12 ] [s 3 () 12 ] [Ir 6 () 16 ] As seen earlier, which is isoelectronic with N 2, has a high-lying axial lone pair at which plays the major role in the coordination with a transition metal. The coordination mode can be η 1, μ 2, or μ 3 η 1 2e terminal µ 3 2e µ 2 2e bridging In some very rare instances, the coordination can also involve the two degenerate π bonds to give 4e and 6e complexes. etal carbonyls are highly fluxional and can adopt several structures in the solid state and in solution. The classical example is [o 2 () 8 ] which displays bridging s in the solid state but none in solution. This difference can be detected by I spectroscopy. o o solid state o solution o solid state: ν() 2071, 2044, 2042, 1866, 1857 cm -1 bridging s 2069, 2055, 2032 cm -1

5 2.2 etal arbonyls 31 In the same vein, the axial and equatorial s of the trigonal bipyramidal [Fe() 5 ] cannot be distinguished by 13 N in solution at room temperature due to their rapid interchange by Berry pseudorotation as discussed earlier. Their separation occurs at 38 in the solid state. As seen previously, has two orthogonal π* accepting orbitals, hence the existence of a strong backbonding in metal carbonyls. This backbonding has several consequences: (1) the bond is lengthened: it increases from Å in free to Å in metal carbonyls; (2) the weakening of the bond can be monitored by I spectroscopy since the stretching frequency is proportional to the square root of the force constant. In free, ν() 2143 cm 1, k = 19.8 mdyne/å, in (terminal) ν() cm 1, k = mdyne/å; (3) the bond is strengthened and becomes shorter: in [e n() 5 ], e n Å, n 1.80 Å. verall, is one of the strongest π-acceptor ligand; the order is: N > > N PF 3 > Pl 3 > P() 3 > P 3 S 2 > N > N 2 2. The bond is relatively weak but its strength varies significantly according to the metal: in [Fe() 5 ], the Fe bond strength is 27.7 kcal mol 1, in [r() 6 ], the r bond strength is 37 kcal mol 1, in [o() 6 ], the o bond strength is 40 kcal mol 1, and in [W() 6 ], the W bond strength is 46 kcal mol 1. As a consequence, metal carbonyls are ideal substrates for substitution reactions. Also, in many cases, clusterization takes place easily by partial loss of under heating or irradiation by UV light: [ ] 70 o2 () 8 [ ] [ ] o() 3 o4 () 12 [ Fe()5 ] hν [ Fe 2 () 9 ] The b est analytical tool to detect metal carbonyls is I spectroscopy. The stretching frequencies are in the range cm 1 for terminal s and cm 1 for μ 2 s. The technique is both very sensitive and very fast (time constants in the range of s.), so that it takes a snapshot of the molecule before it can fluctuate. In so doing, it gives information on the local symmetry of the complex. ere are some indications on the correlation between symmetry and stretching modes. omplex Symmetry ν() modes [Ni() 2 L 2 ] 2v 2 Trans [u 2 () 2 L 2 ] D 2h 2 [pn() 3 ] 3v 2 [Fe() 3 L 2 ] (axial L) D 3h 1 [n() 5 ] 4v 3 Trans [o() 4 L 2 ] D 4h 1 [Ni() 4 ] T d 1 [r() 6 ] h 1

6 32 2 ain Types of rganometallic Derivatives It is possible to understand this correlation between the symmetry of a complex and the number of visible stretching modes on a simple example. Let us consider the cis and trans octahedral complexes [() 2 L 4 ]. The two s can vibrate in phase or out of phase. Thus, two bands would be expected in both cases. But in the trans case, the in phase vibration induces no change in the dipole moment of the molecule and cannot be detected by I. The other technique for the detection of metal carbonyls is 13 N spectroscopy. The carbonyl resonances appear in the range ppm. But this spectroscopy is slow (time constants in the range of 10 1 s.) and gives no reliable information on the local symmetry under standard conditions. For example, at room temperature, the trigonal bipyramidal [Fe() 5 ] gives only one resonance at 210 ppm. 2.3 etal Alkyls and Aryls The metal carbon σ bond is moderately strong (30 65 kcal/mol) but, in general, kinetically unstable because several pathways exist for its decomposition. As a general rule, the orders of thermodynamic stability are >, Ar >, and f(perfluoroalkyl) >. The most efficient decomposition pathway is the β- elimination: In order to confer some kinetic stability to the metal alkyl bond, it is necessary to block this decomposition pathway. There are several possibilities: (1) To use alkyl groups without β-: 3, 2, 2 e 3, 2 Sie 3, 2 F 3, etc. (2) To block the approach of β- to the metal by steric hindrance or geometrical constraint ( = linear) (3) To block the formation of the alkene (when is 1-norbornyl, the alkene cannot form at the bridgehead according to the Bredt rule) (4) To block the formation of the alkene hydride intermediate by using a stable 18e complex that cannot generate the needed vacancy by release of one of its L ligand. For example, in [pfe() 2 Et], both p and are strongly bonded to iron due to a complementary push pull electronic effect and the Fe Et bond is stable. In some cases, even when the β- elimination appears possible (the interaction can be detected either by spectroscopy or neutron diffraction),

7 2.3 etal Alkyls and Aryls 33 the decomposition does not proceed further and the metal alkyl becomes reasonably stable. This type of β- interacting with the metal is called agostic. The species can be viewed as a loose η 2 complex of the bond. agostic The orbital scheme is similar to that of the η 2-2 complexes. In order to stabilize this kind of complexes, it is necessary to suppress the back donation of electrons from the metal to the σ* orbital of the bond. This can be achieved by using d 0 metal centers. This is the reason why metals like Ti +4 and Zr +4 are used in olefin polymerization catalysis and in organic synthesis because both need reasonably stable alkyl complexes as we shall see later. The other decomposition pathways of the bond are the reductive elimination involving alkyl or aryl groups and the α- elimination that leads to carbene complexes as discussed later. The spectroscopic detection of the bond is not as easy as it is for hydrides and carbonyls. With N-active nuclei (spin ½), the 1 J ( ) coupling is useful. The list of convenient metals is given here. Spin 1/2 Isotopic abundance 103 h W s Pt 34 The most useful chemistry of the bonds deals with the insertion of small molecules. Among them, carbon monoxide deserves special attention. The normal scheme is depicted hereafter. L L The stretching frequencies of the resulting acylmetal complexes appear around 1650 cm 1 like for an organic ketone. The situation is different with oxophilic metals from the left of the periodic table:

8 34 2 ain Types of rganometallic Derivatives =Ti(+4),Zr(+4), etc. ere, the products are η 2 -acyl complexes in which both carbon and oxygen are coordinated to the metal. The acyl group acts as a 3e ligand. The stretching frequency is much lower, around 1550 cm 1. The insertion of 2 has also some practical significance. 2 gives an η 2 -complex and the nucleophilic alkyl attacks it at the positive carbon in the vast majority of cases: 2 The attack at oxygen giving () is observed in some rare cases with nonoxophilic metals. The case of S 2 is more subtle because there is still a lone pair at sulfur that is liable to give S 2 complexes. The other coordination mode is η 2 and the final products are either the metallasulfones S() 2 with thiophilic metals (nickel for example) or the sulfinates S() with oxophilic metals (titanium for example). An equilibrium between both types is sometimes observed. S S 2 S S S 2.4 The Zirconium arbon Bond in rganic Synthesis Zirconium combines two features that explain its special role in stoichiometric organic synthesis. It is the most electropositive metal among the transition elements (1.33) at the same level as magnesium (1.31). Its d 0 configuration in its normal +4 oxidation state provides a good kinetic stability to the zirconium carbon bond by disfavoring the β- elimination. The Zr bond is classically obtained by hydrozirconation of alkenes. The zirconium hydride used is generally the easily accessible Schwartz reagent whose synthesis has been mentioned in the section on hydrides. When using a mixture of internal and

9 2.4 The Zirconium arbon Bond in rganic Synthesis 35 terminal alkenes, only the linear alkylzirconium species are obtained, zirconium preferring the terminal position for steric and electronic reasons. With alkynes, the addition of Zr is cis and zirconium occupies the less hindered position. With conjugated dienes, only the 1,2 addition is observed. The zirconium carbon bond easily inserts carbon monoxide and isonitriles but does not react with alkyl halides. The cleavage is easily obtained by reaction with acids (formation of ), bromine, iodine, N-chlororosuccinimide (formation of ) and hydrogen peroxide (formation of ). Zirconium can be replaced by another metal using the reaction with metal halides derived from less electropositive metals (Al, u, Ni). p 2 Zr()l p 2 Zr l p 2 Zr l l Zrp 2 Another reagent of choice for the synthesis of the zirconium carbon bond is the transient zirconocene [Zrp 2 ] obtained by reaction of butyllithium with the stable zirconocene dichloride p 2 Zrl 2. The intermediate p 2 ZrBu 2 loses butane and yields the η 2 complex of zirconocene with butene. This zirconocene reacts with two molecules of alkynes to give a zirconacyclopentadiene. In turn, this zirconacyclopentadiene can serve to prepare boroles, stannoles, siloles, phospholes, arsoles, etc. by metathesis with the appropriate dihalides. p 2 Zrl 2 2 2BuLi p 2 Zr Et =B, Si 2,Sn 2,P, As Zr p 2 Another topic of interest concerns the stabilization of benzyne. This species has been transiently generated by various techniques in organic synthesis. It gives a

10 36 2 ain Types of rganometallic Derivatives stable η 2 complex with zirconocene which is obtained by thermolysis of p 2 Zr 2 at 70 (loss of ) and further stabilized by complexation with e 3 P. This complex is, in fact, a zirconacyclopropene derived from Zr(+4). The complexed bond has the typical length of a = double bond (1.36 Å). It displays a rich chemistry as shown below. 2 1 [Zr] [Zr] =p 2 Zr(Pe 3 ) [Zr] 1 2 = [Zr] 2 = 2 [Zr] 2 N I 2 N [Zr] I I 2.5 etal arbenes etal carbene complexes are schematically divided into two classes, the so-called Fischer carbenes, discovered by Fischer in 1964, and the Schrock carbenes discovered 10 years later by Schrock. The Fischer carbenes are characterized by an electrophilic carbon, whereas the Schrock carbenes are nucleophilic. Both types are conventionally represented as having an = double bond but this representation is misleading. In fact, the best representation of the Fischer carbenes would display a single dative bond 2, the Schrock carbenes being the only ones with a genuine 2 = double bond. In the first case, the carbene acts as an L ligand and the oxidation state of the metal is 0, whereas in the second case, the carbene is equivalent to 2 and the oxidation state of the metal is +2. Before discussing the electronic structures of Fischer and Schrock carbenes, it is necessary to discuss the electronic structure of free carbenes themselves. These 6e species display two frontier orbitals (σ and p) which are shown in the scheme and correspond to the σ and π bond of an alkene which results from the combination of two carbenes.

11 2.5 etal arbenes 37 π bond porbital σ orbital alkene carbene Ep>Eσ σ bond The two electrons of carbon that are not involved in its bonds can occupy the lower σ orbital to give a singlet. Alternatively, each electron can occupy a different orbital to give a triplet. The energies of these two orbitals are close, so the und s rule applies (see the section on L 6 complexes) and states that the triplet is more stable than the singlet because it minimizes the interelectronic coulombic repulsion. triplet carbene singlet carbene In fact, the triplet state of [ 2 ] is more stable than the singlet state by ca. 10 kcal mol 1. The situation changes completely if one (or two) hydrogen is replaced by a lone pair substituent like oxygen or nitrogen. The singlet is strongly stabilized by interaction between the lone pair of the substituent and the vacant p orbital of the carbene whereas no sizeable stabilization occurs in the triplet (one electron is in a destabilized orbital). As a result, the singlet becomes the ground state. triplet carbene no stabilization (3e) singlet carbene stabilized (2e) carbene triplet lone pair carbene vacancy lone pair The Schrock carbenes correspond to the combination of a triplet carbene with a triplet metallic moiety as shown. The z axis is the axis of the double bond and the p orbital of the carbene is in the xz plane. The σ orbital of the carbene overlaps with the d z 2 of the metal to create a σ bond whereas the p orbital of the carbene

12 38 2 ain Types of rganometallic Derivatives overlaps with the d xz orbital of the metal to create a π bond. The is centered on the carbon which is nucleophilic whereas the LU is centered on the metal. The π bond creates a barrier to the rotation around the = bond. LU, mainly localized on, π bond mainly localized on 2 σ bond = 2 The situation is more complex with singlet heteroatom-substituted carbenes. Since the lone pair on the heteroatom is involved, four electrons must be taken into account. The case of [] is illustrated below. The singlet is more stable than the triplet by 17 kcal mol 1. From an orbital standpoint, water and carbene are similar but the oxygen has two lone pairs, one σ and one π. Since is less electronegative than, its frontier orbitals are higher in energy e similar orbitals of and, thus, are destabilized. The overlap between the two σ lone pairs at and is minimal for geometrical reasons. n the contrary, the two π orbitals at and strongly interact since they are parallel. π π π σ σ σ π planeofthe carbene σ The Fischer carbenes typically involve the combination of a singlet carbene like [] with a singlet metal moiety. Since σ is too low in energy to significantly interact with the metal orbitals, it suffices to consider σ, π, and π when building =.

13 2.5 etal arbenes 39 π d zx π σ = σ is stabilized by interaction with d z 2. This orbital corresponds to the dative bond. The d zx orbital is stabilized by the interaction with π and slightly destabilized (poor overlap) by interaction with π. This is mainly centered on the metal and corresponds to some backbonding. The LU is mainly centered on carbon which is electrophilic. The rotation barrier (backbonding) is weak. These orbital schemes depict some limit cases of nucleophilic and electrophilic carbene complex. In practice, a continuum exits between these two limits and the reality is not so clear-cut. As a general rule, Schrock carbenes are derived from high oxidation state metals (Nb(+5), Ta(+5), W(+6)), the = double bond is shorter than the single bond by ca. 10 % and the rotation barriers are in the range kcal mol 1. The Fischer carbenes are derived from low oxidation state metals (r(0), o(0), W(0), Fe(0)), the bond is essentially a single bond and the rotation barriers are very low, ca. 0.5 kcal mol 1. The synthesis of Fischer carbenes supports their theoretical description. The initial synthesis relied on the reaction of organolithium derivatives with metal carbonyls: L n +Nu - L n - Nu E + L n E Nu forexample: L n =W() 5,Nu - =Li and E + =e + (e 3 + ) Since all the chemistry takes place at carbon, the metal must keep its (0) oxidation state. The intermediate acyl anion has a delocalized negative charge between (hard) and (soft), thus a hard alkylation reagent is needed like a trialkyloxonium salt. This chemistry can be transposed to thiocarbonyl derivatives as shown in an example: () 5 W S +NaSe () 5 W S - Se Ie () 5 W Se Se

14 40 2 ain Types of rganometallic Derivatives Acyl derivatives can also be used as shown: L n +E + L n Since the carbon of Fischer carbenes is electrophilic, it can be attacked by nucleophiles. Using this feature, it is possible to exchange the carbene heterosubstituents: L n ' +Nu - L n E ' + Nu L n Nu - = -, -,S -, 2 N - Nu +' It must be noticed that the resulting carbene complex is, in some cases, neither a typical Fischer, nor a typical Schrock carbene complex. These hybrid carbenes have a low stability. The attack of vinyl or alkynyl metal complexes by electrophiles is another route to carbene complexes: e e 2 Still another possibility is the electrophilic abstraction of : F 3 BF 3 F 2 BF 4 - =Fe()(P 3 )p In a very limited number of cases, the reaction of a gem-dihalide with a metal dianion can also afford a carbene complex. In the example shown, it is obviously the 2π aromaticity of the cyclopropenylium cation which drives the formation of the carbene complex which is of the Fischer type (electrophilic) even though it has no heteroatom substituent. Na 2 r() 5 + l l () 5 r The final solution is to use stable free singlet carbenes. ost of them derive from the imidazole ring and are 6π aromatic systems [3]: Ar [(cod) 2 ] Ar Ar Ar =Ni, Pt cod =cyclooctadiene Ar =2,4,6-e Ar Ar

15 2.5 etal arbenes 41 The Schrock carbene complexes are less accessible than the Fischer type. Their main synthesis relies on the α- elimination: Ln Ln 1 2 Since the β- is easier than the α- elimination, the carbon substituent must be devoid of β-. and must be cis. The reaction is favored by steric congestion on the metal since it leads to decompression. This is an intramolecular, first-order reaction. The first step involves a 1,2 migration of from to, followed by a reductive elimination of, hence must be electron deficient. The following example is representative. l 2 pta( 2 e 3 ) 2 14e l 2 pta=e 3 + e 4 The elimination of e 4 is preferred to the elimination of l for thermodynamic reasons. The Ta l bond is much stronger than the Ta bond. The best detection tool for carbene complexes is 13 N spectroscopy. The Fischer carbenes are found in the range ppm (vs. tetramethylsilane), whereas the Schrock carbenes are in the range ppm. There is no correlation between the charge at and the chemical shift. The thermal decomposition of Fischer carbenes yields a mixture of alkenes. () 5 r + is notformed The reaction does not proceed via a free carbene. In the example shown, the free carbene would yield some oxetanone by ring contraction. ild oxidation (air, e(+4), e 2 S, etc.) yields the corresponding carbonyl derivatives: Ln [] [Ln=] +() rganolithium derivatives can perform the exchange of the carbenic heteroatomic substituent as shown earlier. They also can act as a base and abstract a proton located in α position to the carbenic carbon. This reaction is favored over the substitution at low temperature.

16 42 2 ain Types of rganometallic Derivatives () 5 r e 3 BuLi TF, -70 () 5 r e 2 () 5 r e 2-2 () 5 r e The reaction with phosphines can follow different pathways. The initial attack takes place at the electrophilic carbenic carbon as expected: () 5 P 3 () 5 ' ' P 3 Then, either the phosphine migrates onto the metal or a second equivalent of phosphine displaces the Wittig ylide from the metal: () 5 ' P 3 () 4 ( 3 P) ' P 3 () 5 P 3 3 P ' Wittig ylides can be viewed as a source of nucleophilic carbenes. As such, they can be coupled with electrophilic Fischer carbenes to give alkenes: () 5 ' 3 P 2 () 5 P 3 2 ' () 5 3 P 2 ' The reaction with alkenes is very sensitive to the substitution schemes of both reagents. In some cases, the alkene reacts as a nucleophile. The attack takes place at the carbenic carbon and the final product is a cyclopropane. The reaction is stoichiometric.

17 2.5 etal arbenes 43 Ln Ln Ln In some other cases, a metathesis is observed, leading to a new carbene complex and a new alkene. The reaction starts by the complexation of the alkene at the metal and is catalytic. omplexation at the metal Ln -L L (n-1) Formation of a metallacyclobutane L (n-1) L (n-1) L ycloreversion L n Ln The [2 + 2] concerted cycloadditions are forbidden by the Woodward offmann rules, so the question which arises is why it becomes possible with a transition metal. The answer is that the interdiction results from the 4π antiaromaticity of the transition state in the alkene + alkene case. The transition metal switches off the delocalization in the transition state which is not antiaromatic anymore. Thus, the [2 + 2] cycloaddition becomes possible.

18 44 2 ain Types of rganometallic Derivatives alkene +alkene antiaromatic TS alkene +carbene complex d zy not represented for clarity d zx dzx and dzy orthogonal no overlap, no delocalization The reaction with alkynes can also lead to a three-membered ring, especially when the initially formed cyclopropene can aromatize to give a 2π cyclopropenylium ion. The cationic iron carbene complex shown below is so powerful a reagent that it is used in organic synthesis. pfe +2e e -78 o pfe e e e e e e pfe pfe e e As with alkenes, the other possibility is the formation of a four-membered ring. The metallacyclobutene can evolve by ring opening to give a metallabutadiene. With a phenyl substituent on the carbene, the final product is a naphthol. () 5 r + () 5 r () 5 r -[r() 4 ] r() 4

19 2.5 etal arbenes 45 The intermediate ketene complex can be trapped by a nucleophile. () 5 r hν = r() 4 Nu = Nu With an imino substituent on the carbene, a pyrrole can be obtained in good yield through a formal [3 + 2] cycloaddition. () 5 r N Et Pr Pr N Et (95%) Numerous other synthetic applications of Fischer carbenes are known. Such is not the case for Schrock carbenes which are more difficult to prepare and for which the variety of substituents is more limited. Their widest application is as catalysts for the metathesis of alkenes. This point will be discussed later. Apart from that, they behave as super Wittig ylides. The driving force of the Wittig reaction is the formation of the strong P= bond (ca. 130 kcal mol 1 ). With Schrock carbenes, the driving force is much higher: = ca kcal mol 1. The examples given hereafter have no equivalents with Wittig ylides. In practice, only the Tebbe s reagent, a stabilized form of [P 2 Ti= 2 ], has found some use in organic synthesis. ()Et Et e 3 (e 3 2 ) 3 Ta e 3 2 e 3 e 3 ()l (addition) ()Ne 2 e 3 l e 2 N (e 3 2 ) 3 Ta e 3 l (e 3 2 ) 3 Ta e 3 N (e 3 2 ) 3 Ta e 3 (e 3 2 ) 3 Ta N e 3 N 2.6 etal arbynes etal carbyne complexes can be obtained from Fischer carbene complexes by electrophilic abstraction:

20 46 2 ain Types of rganometallic Derivatives +E + +E =e,, l... E + =B 3,Ag + Example: () 5 r e +2Bl 3 () 5 r e +Bl 2 e e Bl - 4 l() 4 r e Another, less versatile method, relies on a double α- elimination promoted by a very high steric congestion on the metal: Wl 6 + 6Li 2 Sie 3 (e 3 Si 2 ) 3 W Sie 3 + 2e 4 Si ther possibilities are the metathesis between and triple bonds and the deprotonation of some carbene complexes =. From a theoretical standpoint, a free carbyne [] has three frontier orbitals, one σ orbital on the z axis and two degenerate orthogonal p orbitals p x and p y of slightly higher energy. Two main electronic configurations are possible. Each of the three available electrons can occupy a different orbital. This configuration behaves as an 3 ligand (S + 3, see the tungsten example before). In the second configuration, two electrons occupy the σ orbital and one a degenerate p orbital. The carbyne behaves as an L ligand (S + 1). In practice, the difference between the two types of carbyne complexes is minimal. The chemistry of carbyne complexes is less developed than the chemistry of carbene complexes. The UV irradiation leads to a transient 1e carbyne complex that can be protonated: hν + ycloaddition onto the triple bond is observed with sulfur, S 2 and [PtL 2 ]. etathesis of alkynes is possible and is now used in organic synthesis. The coupling of two carbynes within a single coordination sphere to give an alkyne has been demonstrated. The deprotonation of a methylidyne complex to give a carbon complex has been achieved.

21 2.6 etal arbynes 47 l 3 (Et 3 P)W t Bu + 70 l 3 (Et 3 P)W + t Bu NEt 2 NEt 2 Br 2-80 o Br 2 NEt 2 =[( 5 e 5 )(EtN)W] + NEt 2 ( 2 N) 3 o 2 K TF [( 2 N) 3 o ] Some epresentative π omplexes The field of π complexes is so enormous that it is impossible to cover it in any detail within the restricted size of this book. A few representative complexes have been chosen for which some applications in organic chemistry have been developed η 4 -Diene-Irontricarbonyls The first complex of this class was discovered in The preparation is quite simple. It suffices to heat a free diene with an iron carbonyl. The more reactive [Fe 2 () 9 ] is preferred to the rather inert [Fe() 5 ]. It is possible to exchange a complexed diene with a more reactive free diene: e e e e Fe() 3 50 o Fe() 3 It is also possible to prepare complexes deriving from dienes that are unstable in the free state using inter alia the reaction of sodium tetracarbonylferrate with the appropriate dihalides:

22 48 2 ain Types of rganometallic Derivatives 2 Br 2 Br Na 2 Fe() Fe() 3 l Fe 2 () 9 Fe() 3 l ontrary to the antiaromatic and unstable free cyclobutadiene, its irontricarbonyl complex is stable and aromatic: it formally derives from the 6π aromatic cyclobutadiene dianion. Its discovery in 1958 was a highlight of transition metal chemistry. Its decomplexation by mild oxidation (e(+4)) has served to generate free transient cyclobutadiene and develop its chemistry. In the butadiene complex, the diene is planar and the iron atom is located on an axis perpendicular to this plane at ca Å. The bonds of the diene are equal at ca Å. The strength of the iron diene bond is average at ca. 48 kcal mol 1. The N data are quite striking. The terminal 2 protons are found at 0.22 (endo) and 1.90 (exo) ppm and the carbon at 41.6 ppm. This huge shielding effect is quite characteristic of the π complexation. The most significant chemistry is the reactivity toward electrophiles. Two pathways are possible, leading either to η 3 -allyl or to functional η 4 -diene complexes. E + E - E Fe() 3 16e Fe() e Fe() 3 E Fe() 3 The reaction gives the η 3 -allyl complex when has a good coordinating ability (l ) and the diene complex when is not a ligand (All 4 ). The acetylation (E=e() All 4 ) of butadiene-irontricarbonyl is 3,850 times faster than the acetylation of benzene. The formylation (E=) by ()N(e) + P()l 3 is also possible. Another point of interest is that, since the two faces of the diene are nonequivalent in these complexes, the terminal carbon atoms become chiral

23 2.7 Some epresentative π omplexes 49 when they are substituted. The complex is, indeed, different from its image as shown. ence, the possibility to develop some applications in enantioselective synthesis has been explored. mirror plane image () 3 Fe () 3 Fe Finally, the easily formed η 5 -coordinated α-carbocations can serve to perform electrophilic substitutions on electron-rich arenes. Fe() 3 Fe() Fe() abstraction Ferrocene The discovery of ferrocene in 1951 was a major turning point of transition metal chemistry. This stable orange solid is easily obtained by reacting [p] ( = Li, Na, gbr ) with Fel 2. All the Fe bonds are equal at 2.06 Å. The two parallel p rings rotate almost freely (barrier ca. 3.8 kj mol 1 ). The N data shows the characteristic shielding associated with π complexation: δ() 4.04, δ() 68.2 ppm (in Dl 3 ). The system is highly aromatic. Both electrophilic acylation and formylation are possible. In fact, the acetylation by e()l + Al l 3 is times faster than that of benzene. Fe e()l All 3 Fe ()e ()N(e) P()l 3 Fe

24 50 2 ain Types of rganometallic Derivatives The 1,1 -diacylation on the two rings is also possible but the 1,1 -diformylation is not. The protonation with strong acids takes place at iron as shown by N: δ() 12 ppm. ono- and dilithiations are feasible: Fe BuLi Et 2 Fe Li TEDA TEDA =e 2 N 2 2 Ne 2 Fe Li Li Aminomethyl substituents direct the metalation toward the α position by chelation of lithium. xidation produces the 17e ferricinium ion. The oxidation is reversible with a midpoint potential E 1/2 of 0.34 V versus SE. The α-ferrocenylcarbocations display a very high stability due to a through-space interaction between iron and the cationic carbon. Finally, a ferrocene with two substituents on the same ring is chiral; this is what is called planar chirality. The combination of a very high stability with a very rich functionalization chemistry explains why ferrocene is such a popular building block for the synthesis of a wide variety of polyfunctional ligands used in homogeneous catalysis η 6 -Arene-hromiumtricarbonyls These species are easily obtained by allowing a free arene to react with chromium hexacarbonyl. The reaction is favored by donor substituents and inhibited by acceptor substituents. In difficult cases, r() 6 is replaced by r() 3 L 3 (L = 3 N, for example). The decreasing stability order is: e 6 6 > e > e 2 N 6 5 >e > 6 6 > e () 6 5 > 6 5 ( = l, F) > 10 8 Naphthalene ( 10 8 ) is the easiest arene to replace and this has synthetic applications. The complexes have the structure of a piano stool (r benzene distance 1.73 Å) and can be characterized by I spectroscopy. They give two bands, the band at low frequency is degenerate and is split when a bulky substituent on the arene breaks the 3v symmetry, for example: ( 6 6 )r() 3 : ν() 1987, 1917 cm 1 ; (e 2 N 6 5 )r() 3 : ν() 1969, 1895, 1889 cm 1.

25 2.7 Some epresentative π omplexes 51 Since the r() 3 group is strongly electron withdrawing, it activates the arene and its substituents toward the attack by nucleophiles. + - r() 3 r() 3 I 2 1) F 3 2 2) I 2 + isomers Decomplexation is achieved by mild oxidation. This scheme has numerous synthetic applications. Some of them are shown here. ( 2 ) 4 N e e N Li -78 o e e ( 2 ) 3 N r() 3 r() 3 I 2 e r() oxidative decomplexation attack on themetaposition for electronic reasons Lithiation needs a directing group in order to be regioselective. F BuLi F Li F r() 3 r() 3 r() 3 I 2

26 52 2 ain Types of rganometallic Derivatives The r() 3 complexing group also activates the benzylic positions, allowing a facile dimetalation: r() e Na Br( 2 ) 3 Br r() 3 2 e The steric bulk of r() 3 can also be used to control the stereochemistry of the reactions on the side chains: (e) 2 N E (e) 2 N Li(e) 2 N E + r() 3 r() 3 r() 3 As in the case of ferrocene, an ortho-disubstituted derivative with two different substituents is chiral. any applications of this planar chirality are possible, especially in enantioselective catalysis. Y Y notsuperimposable with: r() 3 r() Problems II.1 ow would you synthesize [p 2 o( 2 4 )e] + from p 2 ol 2? There is no free rotation of ethylene around its bond with molybdenum. What does that mean? What is the orientation of ethylene with respect to the o e bond? (use arguments based on orbital interactions). ow can you establish this orientation using a classical spectroscopic technique. You have to recall that p 2 ol 2 has a bent metallocene structure. o l l II.2 Propose a mechanism for:

27 2.8 Problems 53 () 5 =() () II.3 The following stoichiometric chemistry has been performed by Barluenga: Li 1 () 5 r e (A) e 3 Si-Tf (B) (B) + 2 () What are the formulas of the chromium complexes (A) and (B) (B does not contain silicon). What are the formula and the two possible stereochemistries of the cyclic organic product () II.4 The reaction of the cationic carbyne complex (A) with a secondary phosphine in the presence of an amine gives the ketene complex (D) (Lugan, rganometallics 2011): n 1 (A) 2 P (B) - + () n P 2 1 (D) (1) Give the formula of the intermediates (B) and (). (2) Propose a mechanism for the last step. What analogy do you find in chromium Fischer carbene chemistry? II.5 p* stands for η 5-5 e 5. (a) Propose a structure for the complex [Irl 2 p*] 2. Discuss the electron count. (b) Upon reaction with NaAc (Ac = acetate), [Irl 2 p*] 2 gives a monomeric complex. Propose a structure for this monomer.

28 54 2 ain Types of rganometallic Derivatives (c) [Irl 2 p*] 2 + NaAc reacts with L 1 to give a single complex A. The 1 N spectrum of A shows two doublets at δ 6.41 and 6.78 ppm due to the pyrrole ring. (d) The same iridium system reacts with L 2 to give complex B. The 1 N spectrum of B shows three doublets at δ 6.38, 6.78 and 7.17 ppm due to the pyrrole protons. Give the structures of A and B and their mechanisms of formation. Note: The complexes are mononuclear (one Ir), the oxidation state of Ir stays identical, the backbone of L is preserved. e N N L 1 =e L 2 = e II.6 (1) A ruthenium catalyst represented by [u] gives two complexes with a primary alkyne. ne is a carbene complex. Give the formula of these two complexes and explain the formation of the carbene complex. (2) The reaction of these two complexes with the carboxylate anion 1 2 yields three products after protonation and loss of [u]: Explain the formation of these products II.7 einekey et al. rganometallics (2011) The reaction of the Ir(I) complex (A) with (B) gives a square pyramidal Ir(III) complex. What are the formula and the number of electrons of ()? Ir (A) N BrAg N AgBr (B) =l, Br N N () Na/g Upon reaction with sodium under hydrogen, () gives the octahedral Ir(III) complex (D). This complex shows a single 1 resonance at 9 ppm (4). What is the formula and the number of electrons for (D)? Why is only one resonance 2 (D)

29 2.8 Problems 55 observed for the hydride? The reaction of (D) with Pe 3 gives another complex (E) with two hydride resonances (1 + 1), whereas the reaction with pyridine gives (F) with a single hydride resonance (2). Why? II.8 The synthesis of an organometallic pyrylium salt has been described (Bruce et al. rganometallics 2011): e ( 3 P)Au Au(P 3 ) [u(dppe)p)] + e - u + u u =u(dppe)p p = η dppe = 2 P- 2 2 P 2 The proposed mechanism involves first a metal exchange to give (A), then a double protonation to give a biscarbene complex (B). The attack of this biscarbene by methylate anion gives a four-membered 3 ring () whose rearrangement gives the pyrylium salt. Give the structures of (A), (B), and () and comment on the various steps. What is the rearrangement that transforms () into the final product? II.9 The following reaction sequence has been described (Alcock et al. rganometallics 1991, 10, 231): t Bu Fe() 3 n BuLi (A) (B) () + n Bu 2 Li t Bu () 3 Fe (A), (B), and () are Fe carbene complexes. Propose a formula for (A), (B) and () and discuss the mechanism for their formation. What reaction described in the book is reminiscent of this transformation?

30 56 2 ain Types of rganometallic Derivatives II.10 eunier, ajoral et al. Angewandte (1997) The following reactions have been performed: p 2 Zr P 80 o toluene (A) (A) +l p 2 Zrl 2 + P What is the formula of (A)? Propose a mechanism for the formation of (A). What is the actual zirconium derivative that reacts with the phosphine? eferences 1. Jessop PG, orris (1992) eactions of transition metal dihydrogen complexes. oord hem ev 121: Elian, offmann (1058) Bonding capabilities of transition metal carbonyl fragments. Inorg hem 1975:14 3. rudden, Allen DP (2004) Stability and reactivity of N-heterocyclic carbene complexes. oord hem ev 248:2247

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